1. Field of the Invention
This invention relates to a control device that controls a self-commutated inverter connected to an AC (alternating-current) power system via a three-phase transformer, and more particularly to an inverter control device that suppresses DC (direct-current) magnetization generated in three-phase transformer.
2. Description of the Related Art
Large capacitance self-commutated inverters have been used to regulate the reactive power of a power system or to supply the DC power from fuel cells or secondary batteries to a power system.
A typical voltage self-commutated inverter is composed of an inverter main circuit, which contains a plurality of controllable rectifier elements and a plurality of rectifier elements, a DC capacitor connected across a DC side terminals of the inverter main circuit, and a three-phase transformer having the inverter side terminals connected to a AC side terminals of the inverter main circuit on a phase basis and the system terminals connected to the system on a phase basis.
Such an inverter is able to control the three phase output voltages of the inverter main circuit by changing ON time of the individual controllable rectifier elements. The current exchanged with the AC power system via the impedance of the three-phase transformer can be controlled by regulating the phase and amplitude of the three phase output voltages from the inverter main circuit according to the phase and amplitude of the AC system voltage.
Such inverter control is performed by an inverter control device. Based on the active current reference value and reactive current reference value, the inverter control device determines an inverter output voltage reference value that decides the three phase output voltages of the inverter main circuit and then supplies a gate signal corresponding to the inverter output voltage reference value to the inverter main circuit to control the ON time of each controllable rectifier element. This converts the DC power from the DC voltage source connected to the DC side terminals of the inverter main circuit into an effective power and supplies it to the AC power system or regulates the reactive power of the AC power system.
The above-mentioned inverter control device has the following problem: a constant generation of a small amount of DC component in the three phase output AC voltages of the inverter main circuit creates a DC component in the alternating magnetic flux in the three-phase transformer, causing an excessive exciting current to flow. That is, DC magnetization in the three-phase transformer gives rise to overcurrent.
When DC magnetization in the three-phase transformer permits a current larger than the rated value to flow in the windings of the three-phase transformer, the transformer may burn out and an excessive current may flow through the controllable rectifier elements constituting the inverter main circuit, which destroys the elements, preventing the voltage self-commutated inverter from functioning properly.
The reason why a small amount of DC component is produced in the three phase output AC voltages of the inverter main circuit is that slight differences in characteristic between the controllable rectifier elements making up the inverter main circuit lead to differences in ON time between the controllable rectifier elements on the positive and negative polarity sides, creating a DC component in the output voltage. The generation of a DC component in the inverter output voltage reference value also causes DC components in the three phase output AC voltages of the inverter main circuit.
Although the problem of DC magnetization in the transformer causing overcurrent can be overcome by providing a gap in the iron core of the transformer to make it hard for magnetic saturation to take place in the iron core, this approach is not desirable because of larger size and higher cost.